Composition and properties of mother-of-pearl

Mother-of-pearl is an primeval material with an iridescent shimmer, which forms the inner coating of certain mollusc shells (oysters, abalone, etc.). It dates back some 500 million years, and its complex composition presents surprising similarities and great complementarity with living tissue, particularly mammals. Like the skeleton of a human embryo formed in the womb, mother-of-pearl is formed by the mollusc in an enclosed space (the extrapallial cavity).

Location of mother-of-pearl

Pinctada Maxima

The shell (exoskeleton) of the bivalve mollusc comprises three layers:

  • An external prismatic calcitic layer
  • A middle layer: interface between calcite and aragonite called the aragonite line
  • An internal layer: aragonitic flat “tablets” running parallel to the shell, bonded to the organic matter

This internal layer is known as mother-of-pearl. Close examination reveals that it has a complex organo-mineral structure at a micrometric and nanometric level, and is composed of aragonite crystals.

Composition of mother-of-pearl

Microstructure of mother-of-pearl observed via electron microscopy

The composition of mother-of-pearl is 95-97% mineral and 3-5% organic.

The close bonding of the mineral and organic structures of mother-of-pearl give rise to an ultrastructure”, whereby the aragonite crystals are the bricks and the organic fraction is the mortar.

95-97% mineral

(Calcium carbonate = CaCO3)

The mineral components of mother-of-pearl are needed for cell and tissue metabolism in mammals.
Almost all of these mineral components are found in mammals.

3-5% organic :

1. Proteins and free amino acids:

Proteins are either essential amino acids (which are not made by the human body, and must be included in the diet) or non-essential amino acids (which are made by the human body). The main amino acids present in the organic fraction of mother-of-pearl play a key role in cell and tissue metabolism, in the formation, growth and repair of the body and play a part in cell ageing.

Proteins can combine with other molecules to form the following:

  • glycoproteins to “growth factor-like”
    In particular, the following glycoproteins are among those contained in mother-of-pearl:

    • Proteoglycans: regulate an array of cellular activities such as cell proliferation, differentiation, adherence and migration.
  • Glycosaminoglycans (or mucopolysaccharides): complexes found in the composition of cartilage and of the hyaluronic acid in conjunctive tissue, among others
    Role: a lubricant that’s a component of the hyaluronic acid present in practically all of the tissues, owing to their high-water retention capacity
  • Metalloproteins: play a role in immune responses
    Role: prompting / accelerating biological reactions
  • Aminoglycosides with “antibiomimetic-like”properties
    Role: antimicrobial

2. Pigments, including:

  • Carotenoids

Role: natural yellow-red colouring pigments, which are responsible for protection against UV rays and the coloration of teguments (skin, nails, hair, claws, fur) + antioxidant.

  • Melanins

Role: dark-coloured biological pigments which are responsible for protection against UV rays and the coloration of teguments (skin, nails, hair, claws, fur) + antioxidant.

3. Lipids and fatty acids (mono- and polyunsaturated)

Role: lubricating, hydrating molecules found in the composition of cells and in the production of certain hormones. They help to retain the cell membrane’s structural and functional integrity

The organic fraction of mother-of-pearl contains substances that stimulate osteogenesis during the formation process (Lopez et al., 1994):

  • Molecules that stimulate the activity of bone tissue, affecting the differentiation of bone-forming cells and mineralisation
  • “Signal” molecules which are active on osteoclastsand can activate osteogenesis of the bone marrow cells (Lamghari et al., 1999)

The organic and inorganic components of mother-of-pearl affect epidermal regeneration and hydration of the skin:

  • Calcium enabling stimulation of the fibroblastsin the dermis (Morvan, 2009)
  • Components involved in cell adhesion and communication
  • Proteins involved in the synthesis of collagen types I and III (Lopez et al., 2000)
  • Lipids to strengthen the epidermal barrier (Rousseau et al., 2006)

Properties of mother-of-pearl

Mother-of-pearl presents multiple properties:

The biological, mechanical and physical properties of mother-of-pearl

The biological properties of mother-of-pearl have particular appeal when it comes to the repair and regeneration.

  • Biocompatibility: refers to a material’s ability to be accepted by the biological environment into which it is implanted, and to not cause damage to it. The by-products of the breakdown of mother-of-pearl (water, CO2, Ca2+ are not toxic to the human body
  • Angiogenesis: the physiological process of forming and growing new blood vessels governed by growth factors

The mechanical properties of mother-of-pearl have particular appeal when it comes to the repair and regeneration of hard tissue (bone and cartilage):

  • Mechanical strength: outperforming bone, for example, hence the benefit of working on an alternative, with dental implants made of mother-of-pearl
  • Elasticity: mother-of-pearl outperforms human bone in terms of elasticity

The physical properties of mother-of-pearl, too, are of particular appeal when it comes to the repair and regeneration of hard tissue (bone and cartilage):

  • Osteoconduction: the physiological process enabling circulation of the cell elements needed to repair a loss of bone substance. Mother-of-pearl in powdered form, with set granulometric parameters, promotes the porosity needed for the circulation and proliferation of new vessels and of the cell elements needed to heal damaged tissu

Here are two of mother-of-pearl’s most remarkable properties:

  • Entering systemic local regulation mode

Mother-of-pearl’s property enables the tissues in contact with it to get all the properties and functions that they need: no risk of overdose (the tissues only use what they need or excessive proliferation. 

  • Osteomimetic behaviour

To date, mother-of-pearl is the only known biomaterial that is osteomimetic:

    • Behaves like an autologous graft(1)
    • Places itself under the control of the systemic regulation(2) of the recipient site (the so-called beneficiary)
    • Has a mineral-based organic component that imparts mechanical properties to it which are similar to those of bone
    • Has unique angiogenesis properties(3)

(1) A graft whereby the donor and recipient are the same person
(2) Ability to adapt to the characteristics of the recipient site
(3) The physiological process of formation and proliferation of new blood vessels

The osteomimetic behaviour of mother-of-pearl